Refactor sim initialisation to fit G4 standard methods
Building on !43030 (closed)
Uses the changes in !43995 (merged)
Geant4 has standard methods like ConstructSDAndFields that initialise the detector, with appropriate per-thread behaviour. However, we don't use these methods very well at present. This MR attempts to move Athena simulation methods into the corresponding G4 methods.
Edited by Benjamin Michael Wynne